The various embodiments of the present invention relate to self-hardening calcium phosphate-containing and/or calcium-containing paste and cement compositions. The compositions may be used to form pastes for bone and tooth restoration and similar applications, where the paste will harden within a desired time after being delivered to a repair site.
Most conventional calcium phosphate cements are mixed with an aqueous solution immediately before application. In the clinical situation, the ability of the surgeon to properly mix the cement and then place the cement paste in the defect within the prescribed time is a crucial factor in achieving optimum results.
A self-hardening calcium phosphate cement (CPC), consisting of tetracalcium phosphate (Ca4(PO4)2O, also referred to as “TTCP”) and dicalcium phosphate anhydrous (CaHPO4, also referred to as “DCPA”), has been shown in clinical studies to be efficacious for repairing bone defects. The hardening time of such conventional cements is as long as about 30 minutes with water, although hardening time can be shortened if a phosphate solution is used as the cement liquid. Hydroxyapatite (Ca5(PO4)3OH, also referred to as “HA”) is formed as the product. More recently, additional CPCs that do not contain TTCP, e.g., α-tricalcium phosphate (α-Ca3(PO4)2, also referred to as “α-TCP”) and CaCO3 or DCPA and Ca(OH)2, have also been developed. These cements may harden in about 10 minutes when a phosphate solution is used as the cement liquid. They also form hydroxyapatite as the final product.
A premixed CPC paste containing the TTCP and DCPA powders and glycerol as the cement liquid has been used for root canal filling and sealing by injection techniques. The cement paste was found to be stable in a syringe but hardened only after being delivered into the root canal where it became exposed to water from the surrounding tissues. Because the cement paste was injected into a confined area, there was little concern of disintegration of the paste due to washout. Although the premixed CPC was shown to have improved biocompatibility with periapical bone tissue than a number of conventional root canal filling or sealing materials, the premixed CPC-glycerol paste did not exhibit a good washout resistance when it was applied to an open wet field.
The patent literature also describes at least one class of calcium phosphate cement compositions which are precursors for the formation of hydroxyapatite and are biologically compatible, and have two unique properties that are not attainable in other calcium phosphate biomaterials: (1) self-hardening to form a mass with sufficient strength for many medical and dental applications, and (2) when implanted in bone, the cement resorbs slowly and is completely replaced by new bone formation with no loss in the volume or integrity of the tissue that receives the implant. See U.S. Pat. Nos. Re. 33,221 and Re 33,161 to Brown and Chow, which teach preparation of calcium phosphate remineralization compositions and of finely crystalline, non-ceramic, gradually resorbable hydroxyapatite cement based on the same calcium phosphate compositions.
The major components of the calcium phosphate remineralizing slurries, pastes and cements taught in U.S. Pat. Nos. Re. 33,221 and Re. 33,161 are preferably tetracalcium phosphate (Ca4(PO4)2O), and at least one other sparingly soluble calcium phosphate, preferably dicalcium phosphate anhydrous (CaHPO4), or dicalcium phosphate dihydrate (CaHPO4.2H2O). These react in an aqueous environment to form hydroxyapatite, the principal mineral in teeth and bones, as the final product. Because of the apatitic nature of the set cement, it is highly compatible with soft and hard tissues. This material, if applied intraoperatively as a paste, subsequently sets to a structurally stable implant composed of microporous hydroxyapatite.
A virtually identical calcium phosphate system which consists of tetracalcium phosphate (TTCP) and monocalcium phosphate anhydrous (MCPA) or its monohydrate form (MCPM) was described by Constantz et al. (U.S. Pat. Nos. 5,053,212 and 5,129,905). This cement system is believed to involve conversion of the MCPA to dicalcium phosphate which reacts with TTCP and forms hydroxyapatite, the major mineral component of teeth and bone, as the end product.
Constantz et al. U.S. Pat. Nos. 4,880,610 and 5,047,031 describe another cement system that consists of a mixture of solid phosphoric acid crystals, calcium carbonate, and calcium hydroxide as the cement powder and a 7.4 mol/L NaOH solution (4.5 g NaOH in 15 mL of water) as the cement liquid. Data on the physical and chemical properties (compressive strength, hardening time, nature of end product, pH of the cement fluid, heat of mixing etc.) of this cement have not been located in the patent or scientific literature.